Display devices, such as touch panels and liquid crystal displays, have problems such that when the copper wiring circuit disposed below the liquid crystal layer has a high reflectance, the influence of the reflection causes a reduction in the appearance of the display device, and deterioration of the display accuracy. Therefore, a reduction in the reflectance by blackening copper circuits and conductor parts comprising other copper materials is required.
As a method for blacking a copper circuit, there is, for example, a known method in which when a printed wiring board is laminated, an uneven oxide film is formed on the surface of a copper circuit formed on an inner layer board to ensure adhesion between the copper surface and a prepreg resin to be laminated. This method is called “blackening treatment,” whereby a copper surface is oxidized using hypochlorite, chlorite, or the like in an alkali solution containing a stabilizing agent to form a copper oxide film. There is another method for blackening a copper surface with an aqueous sulfide solution (see PTL 1, described later).
However, the above method for oxidizing a copper surface cannot sufficiently reduce the reflectance because the degree of blackening is not sufficient. Further, sulfide treatment has a problem such that the surface and side of a copper circuit are roughened, reducing wiring accuracy (see PTL 2, described later).
Moreover, in recent years, a method for forming a circuit using conductive silver paste has been implemented as a relatively simple method for forming a wiring circuit. When such a wiring circuit made of silver paste is used in display devices, such as touch panels and liquid crystal displays, a reduction in reflectance by blackening a conductor part comprising silver as a main component is also required.